Delivery device for delivering a liquid additive out of a tank, method therefor and motor vehicle having a delivery device

ABSTRACT

A delivery device for delivering a liquid additive out of a tank to an exhaust-gas treatment device includes a delivery duct with an overall volume and a pump disposed in the delivery duct. The delivery duct has a flexible wall portion, downstream of the pump in a delivery direction, with an outer side of the flexible wall portion situated opposite the delivery duct and bearing against a stop when a pressure in the delivery duct lies in a predefined operating pressure range. A spring element which is disposed on the outer side is constructed to deform the flexible wall portion in such a way that the overall volume is reduced in size when the pressure in the delivery duct is lower than a threshold pressure. A method for compensating a formation of ice in a delivery device and a motor vehicle having a delivery device are also provided.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation, under 35 U.S.C. §120, of copending International Application No. PCT/EP2012/071530, filed Oct. 31, 2012, which designated the United States; this application also claims the priority, under 35 U.S.C. §119, of German Patent Application DE 10 2011 118 628.3, filed Nov. 16, 2011; the prior applications are herewith incorporated by reference in their entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The invention relates to a delivery device for delivering a liquid additive out of a tank to an exhaust-gas treatment device. Exhaust-gas treatment devices to which a liquid additive is supplied are widely used, in particular, in the automotive field. An exhaust-gas purification method particularly commonly implemented in such exhaust-gas treatment devices is the selective catalytic reduction (SCR) method in which nitrogen oxide compounds in the exhaust gas are converted with the aid of a reducing agent to form non-harmful substances (for example water, nitrogen and/or CO₂). The reducing agent is often supplied to such exhaust-gas treatment devices in the form of a liquid additive, in particular in the form of a urea-water solution. A urea-water solution with a urea content of 32.5 percent is available for that purpose under the trademark AdBlue®. The invention also relates to a method for compensating the formation of ice in a delivery device and a motor vehicle having a delivery device.

In order to supply the liquid additive to the exhaust-gas treatment device, a delivery device is generally required which transports the liquid additive from the tank to the exhaust-gas treatment device. A delivery device should operate with the highest possible dosing accuracy, should be as inexpensive as possible and should have as long a service life as possible.

If reducing agent is used as the liquid additive, it is a technical problem that such aqueous reducing agents freeze at temperatures that may arise during normal operation of a motor vehicle.

The urea-water solution AdBlue® commonly used as reducing agent for exhaust-gas purification freezes, for example, at −11° C. In the case of motor vehicles, such low temperatures may arise, in particular, during long standstill periods in winter. As a liquid additive freezes, the additive generally increases in volume. That increase in volume can damage the delivery device. A delivery device should therefore, if appropriate, be constructed in such a way that it is not damaged by an increase in volume of the additive and by the associated ice pressure that is generated.

SUMMARY OF THE INVENTION

It is accordingly an object of the invention to provide a delivery device for delivering a liquid additive out of a tank, a method therefor and a motor vehicle having a delivery device, which overcome the hereinafore-mentioned disadvantages and solve, or at least lessen, the highlighted technical problems of the heretofore-known devices, methods and vehicles of this general type as advantageously as possible. It is sought, in particular, to describe a particularly advantageous delivery device for delivering a liquid additive. Furthermore, it is sought to specify a particularly advantageous method for compensating the formation of ice in a delivery device.

With the foregoing and other objects in view there is provided, in accordance with the invention, a delivery device for delivering a liquid additive out of a tank to an exhaust-gas treatment device, comprising at least a delivery duct with an overall volume and a pump disposed in the delivery duct, the delivery duct having, downstream of the pump in a delivery direction, a flexible wall portion which, with an outer side situated opposite the delivery duct, bears against a stop when the pressure in the delivery duct lies in a predefined operating pressure range. A spring element which is provided on the outer side is constructed to deform the flexible wall portion in such a way that the overall volume of the delivery duct is reduced in size when the pressure in the delivery duct is lower than a threshold pressure.

The delivery device preferably has a suction point at which the liquid additive can be suctioned out of a tank. A tank typically has an interior space, in which the liquid additive is stored, and a tank wall, which has a tank base and a tank top side and which delimits the interior space. The suction point either may be in direct contact with the interior space of the tank or is connected through a line piece to the interior space of the tank.

The delivery device furthermore preferably has an outlet port. The liquid additive is provided at the outlet port. The outlet port is preferably connected through a line to an injection device. The injection device may be disposed on an exhaust-gas treatment device and constructed to supply the liquid additive into the exhaust-gas treatment device. The injection device may include a nozzle for injecting the liquid additive into the exhaust-gas treatment device and/or a valve for controlling the amount of additive supplied into the exhaust-gas treatment device.

In the described delivery device, preferably reducing agent (or a reducing agent precursor which forms ammonia) and particularly preferably aqueous urea solution is used as liquid additive. The method of selective catalytic reduction is carried out using the reducing agent in an exhaust-gas treatment device.

The delivery device is preferably disposed directly on/in the tank for storing the liquid additive. The delivery device is preferably disposed in a separate chamber disposed on or at least partially in the tank. The exhaust-gas treatment device preferably includes, in addition to the injection device, an SCR catalytic converter in which the SCR method can be carried out with the aid of a reducing agent which is provided as the liquid additive to the exhaust-gas treatment device.

The delivery duct preferably forms a portion of a flow path for the liquid additive from the tank to the exhaust-gas treatment device. The overall volume of the delivery duct preferably refers to the volume which the delivery duct has in the delivery device and which is filled with the liquid additive during the operation of the delivery device. The overall volume refers, in particular, to the volume which the delivery duct has downstream of the pump in the delivery direction of the liquid additive and which is filled with liquid additive. The pump is connected to the delivery duct. The pump preferably forms a portion of the delivery duct. The pump divides the delivery duct into a suction portion from the suction point to the pump and a pressure portion from the pump to the outlet port. The overall volume may, in particular, be formed only by the pressure portion of the delivery duct. It is preferable for at least one valve to be provided in the pump, which valve predefines a delivery direction of the liquid additive through the pump. For this reason, the liquid additive cannot flow through the pump counter to the delivery direction. Furthermore, the pump has a movable pump element which can perform a delivery movement. Due to the delivery movement, the liquid additive is delivered through the pump.

The delivery duct has a duct wall. The duct wall is preferably formed for the most part in the manner of a bore in a plate. The flexible wall portion is preferably not fixedly connected to the plate, or not a fixed constituent part of the plate. Those wall portions of the delivery duct which are formed by the plate are, on the whole, rigid. The flexible wall portion is preferably composed of a flexible material. The flexible wall portion may be formed by a flexible diaphragm, for example. The flexible wall portion or the flexible diaphragm may be formed, for example, from plastic, in particular from rubber. The flexible wall portion is preferably reversibly/elastically deformable. The flexible wall portion or the flexible diaphragm is preferably formed by an areal element. The areal element has a first surface and a second surface. The first surface is directed toward the delivery duct and forms the flexible wall portion. The second surface forms the outer side which is situated opposite the delivery duct and the first surface. The stop against which the opposite outer side bears is formed, for example, by a cap element or a cover which makes contact with the outer side. The spring element is preferably disposed between such a cap and the outer side of the flexible wall portion.

The operating pressure range lies preferably at pressures above 3 bar. The operating pressure range lies particularly preferably between 5 bar and 10 bar and very particularly preferably between 7 bar and 9 bar. The spring element is constructed to push or press the flexible wall portion into the delivery duct or into the overall volume when the pressure in the delivery duct is lower than a threshold pressure. The threshold pressure lies preferably below the lower limit of the operating pressure range, and therefore preferably below 3 bar. The flexible wall portion may have a construction which promotes a deformation. For example, the flexible wall portion may have provided on it corrugations which promote a deformation of the flexible wall portion.

In accordance with another particularly advantageous feature of the delivery unit of the invention, a return line branches off from the delivery duct downstream of the pump in the delivery direction (in the region of the pressure portion of the delivery duct), which return line can be closed off by a return valve. The return line branches off from the pressure portion in order to be able, by using the return valve, to ensure a release of pressure from the pressure portion if appropriate. A return flow of liquid additive through the pump is prevented by the valve provided in the pump, as described further above. A release of pressure from the pressure portion of the delivery duct through the pump is therefore prevented. A release of pressure from the pressure portion through the outlet port would lead to a loss of liquid additive, because the additive discharged through the outlet port passes to the injection device. A loss-free release of pressure is possible, in particular, through the return line. When the flexible wall portion deforms, the liquid additive which is displaced out of the delivery duct due to the reduction in size of the overall volume can escape through the return line.

In accordance with a further particularly advantageous feature of the delivery unit of the invention, disposed on the stop is at least one force sensor which can measure the force exerted on the stop by the flexible wall portion. A pressure in the delivery duct can thus be determined. The flexible wall portion maintains its flexibility even when it is bearing against the stop. The flexible wall portion then transmits (at all points) the force exerted by the pressure in the delivery duct directly to that region of the stop which is situated (directly) opposite. Through the use of a force sensor integrated in the stop, the situation can be utilized in order to carry out a measurement of the pressure in the delivery device or in the delivery duct of the delivery device. The pressure sensor is preferably integrated into the stop so as to end flush with the stop. The force sensor may also protrude slightly beyond the stop in order to ensure that a force exerted on the flexible wall portion is transmitted more easily to the force sensor. During operation (when the flexible wall portion bears against the stop), the outer side of the flexible wall portion presses against the force sensor. The force actually acting on the force sensor is also defined by the surface area of the force sensor (or the surface area of the flexible wall portion pressing against the force sensor). The surface area of the force sensor is multiplied by the pressure in the delivery duct in order to calculate the force exerted. The surface area of the force sensor is preferably relatively small in relation to the overall surface area of the stop against which the flexible wall portion bears. The surface area of the sensor amounts to preferably at most 1/10, and particularly preferably at most 1/20, of the overall surface area of the stop. Conventionally used force sensors measure an exerted force on the basis of a change in length or a deformation of an elastic material with a known modulus of elasticity. Therefore, a slight deformation of the flexible wall portion takes place at the force sensor even in the presence of the operating pressure in the delivery duct because the flexible wall portion deforms, in the region of the sensor, toward the sensor so as to transmit a force from the outer side to the force sensor. The deformation is preferably as small as possible in order to ensure a small change in the overall volume of the delivery duct within the operating pressure range.

The force sensor is preferably formed by a piezo material and/or by a deformable (electrical) resistance. A piezo material generates a different voltage according to how intensely it is deformed. In the case of a deformable resistance, the electrical resistance changes as a function of a deformation. The deformation is in each case proportional, based on the modulus of elasticity, to the acting force. The generated electrical voltage is in each case proportional to the deformation. An acting force can thus be converted into electrical voltage and evaluated by electronics.

In accordance with an added particularly advantageous feature of the delivery unit of the invention, the pump has a pump outlet and the delivery duct has a chamber downstream of the pump outlet in the delivery direction, the pump outlet opens into the chamber, and the flexible wall portion is disposed on the chamber opposite the pump outlet.

The chamber is, in particular, an extension of the delivery duct. The chamber is preferably formed as a recess which is situated on a plate of the delivery device. A portion of the delivery duct which extends from the pump outlet opens into the chamber. A further portion of the delivery duct which extends to the outlet port branches off from the chamber. Since the chamber is provided as a recess in the plate, the chamber can be covered by an areal element which forms the flexible wall portion. This yields a particularly simple possibility for mounting the flexible wall portion of the delivery duct. A cap which forms the stop can then be inserted in the described recess in the plate, at the outer side of the flexible element or material which forms the flexible wall portion.

The pump outlet of the pump is preferably aligned along a common axis with a pump inlet of the pump. During delivery operation, the additive exits the pump outlet as a pulsating flow with pressure fluctuations. The described chamber is also advantageous from a flow aspect. The pulsating flow from the pump outlet is homogenized in the chamber. This arises, in particular, due to the enlarged volume of the chamber and/or the diversion of the flow which occurs when the additive exits the chamber again.

The chamber and the flexible wall portion are preferably disposed spatially on a common axis with the pump inlet and the pump outlet. An axis running through the pump inlet and the pump outlet preferably intersects the chamber and the flexible wall portion. The chamber is delimited by the flexible wall portion preferably on a side situated opposite the pump outlet or the pump.

In accordance with an additional particularly advantageous feature of the delivery unit of the invention, the stop has a receptacle in which the spring element is accommodated when the flexible wall portion bears against the stop.

Such a receptacle makes it possible for the spring element to behave rigidly when the pressure in the delivery duct lies in the operating pressure range. The spring element is then accommodated entirely in the receptacle and, in this case, is preferably compressed. If the stop is formed by a cap, the receptacle may be formed as a recess of the cap. For a spring element constructed as a spiral spring, the receptacle may be formed, for example, as an annular groove in the cap, the diameter of which groove corresponds to the diameter of the spring and the depth of which groove is sufficient to completely accommodate the spring element in the compressed state.

In accordance with yet another particularly advantageous feature of the delivery unit of the invention, the spring element exerts on the flexible wall portion a spring force which corresponds to a pressure of 0.2 to 1.0 bar in relation to the surface area of the flexible wall portion when the flexible wall portion bears against the stop. Through the use of a force corresponding to such a pressure, the liquid additive can be forced out of the overall volume of the delivery duct of the delivery device even if a certain counter-pressure is acting on the delivery device from the outside. The liquid additive should preferably be forced through a return line back into a tank. The delivery device is preferably disposed on the base of a tank. The pressure acting on the delivery duct or on the overall volume is defined substantially by the filling level in the tank if the delivery device is disposed on the tank base and a return valve in a return line which connects the delivery duct to the tank interior space is open. A pressure of 0.2 to 1.0 bar then corresponds to the threshold pressure above which the overall volume in the delivery duct is reduced in size. The threshold pressure is preferably selected in such a way that an adequate spacing from the operating pressure range is obtained and it is thus ensured, that the delivery device behaves substantially rigidly when the pressure lies in the operating pressure range and a reduction in size of the overall volume occurs only when an operational stoppage of the delivery device actually takes place and the return valve in a return line is open. The spring force of the spring element required to ensure the above-specified values for the threshold pressure may be determined and fixed on the basis of the desired threshold pressure and the surface area of the flexible wall portion on which the spring element acts and which is exposed to the pressure in the delivery duct.

The outer side of the flexible wall portion is preferably connected to the environment through an exchange duct in such a way that a pressure acting on the outer side of the flexible wall portion does not exert a force which distorts the behavior (in particular the pressure-deformation characteristic curve) of the flexible wall portion.

A force exerted by the spring element on the flexible wall portion is, if appropriate, also taken into consideration in the determination of the pressure in the delivery duct. A part of the pressure in the delivery duct may be compensated by the spring element in such a way that the force sensor can measure only a part of the pressure. In order to be able to measure the pressure in the duct, it is necessary, if appropriate, to take into consideration that pressure component of the pressure measured by the force sensor which is compensated by the spring element.

With the objects of the invention in view, there is furthermore provided a method for compensating the formation of ice in a delivery device having an overall volume of a delivery duct being at least partially filled with liquid additive, wherein the method comprises at least the following steps:

-   -   a) stoppage of the operation of a pump of the delivery device;     -   b) opening of a return valve of a return line which produces a         connection between the delivery duct and a tank for the liquid         additive;     -   c) active reduction in size of the overall volume filled with         liquid additive;     -   d) discharging of liquid additive through the return valve into         the tank; and     -   e) passive increasing of the overall volume as the liquid         additive in the delivery duct freezes.

The method according to the invention may be implemented or carried out, in particular, with the delivery device according to the invention. The advantages and structural features explained with regard to the described delivery device can be transferred analogously to the described method. The same applies to the advantages and special configuration features of the method according to the invention described below, which can be transferred analogously to the delivery device according to the invention.

A compensation of the formation of ice refers in this case, in particular, to a compensation of the volume expansion which normally arises in conjunction with the formation of ice, or of the generated ice pressure of the additive. The volume expansion is obtained by an increase in size of the volume provided for the additive.

The stoppage of the operation of a pump in step a) is usually associated with the stoppage of an internal combustion engine connected to an exhaust-gas treatment device, to the exhaust-gas treatment device of which internal combustion engine the delivery device is connected in order to supply the liquid additive.

The opening of the return valve in a return line in step b) normally takes place automatically. The return valve is preferably constructed as a solenoid valve which is closed when an electrical current is applied and which is open when no electrical current is applied. Upon the stoppage of operation, an electrical current supply to the delivery device is preferably interrupted. The return valve in the return line then opens automatically and produces a connection between the overall volume of the delivery duct and a tank through the return line. As a result of this, the pressure in the overall volume falls because the pressurized liquid additive in the delivery duct or in the overall volume can escape through the return line.

In step c), the overall volume can then be reduced in size with relatively little force. In this case, an active reduction in size means that a component of the delivery unit is provided for initiating or carrying out a process of reducing the size of the overall volume. This may be realized, for example, by a spring which deforms a flexible wall portion of the delivery duct into the delivery duct. It is, however, also possible, in a further embodiment of the method, for a mechanically actuable device (if appropriate with an electric drive) to actively reduce the size of the overall volume. For example, a slide element may be moved into the overall volume in order to reduce the size of the overall volume.

As a result of the deformation, the liquid additive is, in step d), forced through the return valve and through the return line into the tank. There is a temporary flow of the liquid additive out of the overall volume through the return line and back into the tank.

As long as the temperature (to which the delivery device is exposed) does not fall far enough to cause liquid additive to solidify into ice, the overall volume of the delivery duct does not change. If the temperature falls further and the formation of ice in the delivery duct of the delivery device begins, the overall volume increases in size again in step e) to the extent required for compensating the freezing. In this case, a passive increase in size means, in particular, that the increase in size is not realized by an actively moved component of the delivery unit but rather is dependent on external circumstances (in the present case, the volume expansion of water-based additives when freezing occurs). The compensation of the freezing or of the volume expansion which takes place when freezing occurs preferably takes place again (only) by a deformation of the flexible wall portion. The deformation which occurs in step e) preferably opposes the active deformation provided in step c).

For clarity, it is pointed out in this case again that (if this does not emerge from the above description itself) steps a)-d) take place in the specified sequence when freezing occurs.

With the objects of the invention in view, there is concomitantly provided a motor vehicle, comprising an internal combustion engine, an exhaust-gas treatment device for purification of exhaust gases of the internal combustion engine, a tank for a liquid additive, and a delivery device described according to the invention which is constructed to deliver the liquid additive from the tank to the exhaust-gas treatment device.

The delivery device of the motor vehicle is particularly advantageously also suitable for carrying out the described method.

Other features which are considered as characteristic for the invention are set forth in the appended claims, noting that the features specified individually in the claims may be combined with one another in any desired technologically meaningful way and may be supplemented by explanatory facts from the description, with further structural variants of the invention being specified.

Although the invention is illustrated and described herein as embodied in a delivery device for delivering a liquid additive out of a tank, a method therefor and a motor vehicle having a delivery device, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.

The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a diagrammatic, vertical-sectional view of a delivery device;

FIG. 2 is a further vertical-sectional view of a delivery device;

FIG. 3 is a block diagram of a motor vehicle;

FIG. 4 is a diagram of a pressure in a delivery device; and

FIG. 5 is a vertical-sectional view of another structural variant of a delivery device.

DETAILED DESCRIPTION OF THE INVENTION

Referring now in detail to the figures of the drawing for explaining the invention and the technical field in more detail by showing particularly preferred structural variants to which the invention is not restricted and in which size ratios are diagrammatic, and first, particularly, to FIGS. 1 and 2 thereof, there are seen different aspects of a delivery device 1. A joint explanation of FIGS. 1 and 2 will therefore be given first herein. Illustrated in each case is the delivery device 1, having a delivery duct 4 which extends from a suction point 17 to an outlet port 18. A pump 5, which is provided in the delivery duct 4, delivers the liquid additive through the delivery duct 4. A chamber 16 is situated downstream of the pump 5 and downstream of a pump outlet 15, in a delivery direction 6 illustrated in FIG. 1. The chamber 16 is a constituent part of the delivery duct 4. The delivery duct 4 is disposed together with the chamber 16 in a plate 19. The delivery duct 4 and the chamber 16 may, for example, be drilled or (if the plate 19 is formed as a cast part) cast into the plate 19. The chamber 16 is delimited, on the side situated opposite the pump outlet, by a flexible wall portion 7. The flexible wall portion 7 therefore also forms a wall portion of the delivery duct 4. The flexible wall portion 7 has a surface 23 through which it is in contact with the delivery duct 4 and with the chamber 16. The delivery duct 4 together with its chamber 16 has an overall volume 20. The flexible wall portion 7 is supported, on an outer side 29, against a stop 9. The stop 9 may be formed, for example, by a cap which covers the flexible wall portion 7. A force sensor 14 may also be provided in the stop 9 for determining a pressure in the delivery duct 4 or in the chamber or in the overall volume 20. A spring element 8 is situated between the flexible wall portion 7 and the stop 9. A receptacle 30 is preferably provided in which the spring element 8 can be accommodated when the flexible wall portion 7 bears against the stop 9. The surface 23 of the flexible wall portion 7 is definitive for the force exerted on the spring element 8 by the pressure in the delivery duct 4. A return line 26 preferably branches off from the delivery duct 4 and leads back into a tank for the liquid additive. The return line 26 can be opened and closed by a return valve 21.

FIG. 1 illustrates the delivery device 1 when an operating pressure is prevailing in the overall volume 20. The flexible wall portion 7 then bears against the stop 9.

FIG. 2 illustrates the delivery device 1 when the pressure in the overall volume 20 is lower than a threshold pressure. The spring element 8 is then expanded and the flexible wall portion 7 is deformed into the overall volume 20 or into the chamber. The overall volume 20 is therefore reduced in size in FIG. 2.

FIG. 3 shows a motor vehicle 24 having an internal combustion engine 25 and having an exhaust-gas treatment device 3 for the purification of the exhaust gases of the internal combustion engine 25. An injection device or injector 28, with which liquid additive can be supplied into the exhaust-gas treatment device 3, is provided on the exhaust-gas treatment device 3. For this purpose, liquid additive is supplied to the injection device 28 from a tank 2 through a line 27 and a delivery device 1.

FIG. 4 shows a diagram of a pressure in a delivery device. The pressure in the delivery device is plotted on the horizontal axis 13. The overall volume of the delivery device is plotted on a vertical volume axis 12. It is possible to see the operating volume which is constant in an operating pressure range 11 and which, during regular operation of the delivery device, forms the overall volume and is present in the delivery device. If the pressure falls below a threshold pressure 31, the overall volume decreases. The threshold pressure 31 denotes the pressure beyond which an expansion of the spring element occurs. The curve depicting the relationship between the pressure and the overall volume has a further bend below the threshold pressure 31. The bend denotes the pressure at which an expansion of the spring is complete. Furthermore, the spring cannot press the flexible wall portion into the delivery duct.

FIG. 5 shows a further structural variant of a delivery device 1. The delivery device 1 also has a base plate 19 in which a delivery duct 4 for the delivery of liquid additive is situated. In this case, too, a pump 5 forms a portion of the delivery duct 4. It is also possible to see a return line 26 which branches off from the delivery duct 4 and which can be opened and closed by a return valve 21 in order to produce or break a connection from the delivery duct through a return line back into a tank. In the structural variant of a delivery device according to FIG. 5, the delivery duct 4 is constructed in such a way that horizontal portions (wherein horizontal is defined in relation to a preferred installation alignment of the delivery device in a motor vehicle) in each case have an angle of inclination 22 with respect to a horizontal alignment. In this way, it is possible to prevent air bubbles from becoming trapped in the ducts. Such air bubbles can be conveyed out of the delivery device or out of the delivery duct only by using an increased pressure, in particular also due to capillary forces. The angle of inclination 22 is preferably selected in such a way that a flow path for an air bubble from any point of the delivery duct 4 to the return line 26 or to the return valve 21 is monotonously rising. The angle of inclination 22 is preferably even selected in such a way that such a flow path for an air bubble to the return valve 21 is monotonously rising even when the delivery device is in a slightly oblique position. Such an oblique position may arise during operation, for example, because the vehicle with the described delivery device is parked in an oblique position. The angle of inclination 22 is preferably at least 2° and particularly preferably at least 5°. It is thus possible for oblique positions of up to 2° or even up to 5° to be compensated.

The concept of providing delivery ducts in a delivery device with an angle of inclination may also be implemented independently of the concept, described further above, of a delivery device having a flexible wall portion. In particular, there is also specified herein a delivery device having at least one delivery duct and having a pump for delivering reducing agent from a suction point in a tank to an outlet port, in which all of the ducts in the delivery device are at an angle of at least 2°, in such a way that from any point of the delivery duct, a flow path for an air bubble to a return valve exists which (in terms of the geodetic position of the air bubble) rises monotonously. Such a delivery device may be supplemented as desired with further features from the description. It is thereby possible, specifically for the field of application mentioned in the introduction, for the formation of gas bubbles in the ducts to be reduced, or for the migration of the gas bubbles in the delivery unit to be influenced in a targeted manner. It is thereby possible to obtain a reduction not only in pressure fluctuations but also, for example, in malfunctions in the delivery of the heating of frozen reducing agent and/or servicing and monitoring measures. 

1. A delivery device for delivering a liquid additive out of a tank to an exhaust-gas treatment device, the delivery device comprising: a delivery duct having an overall volume; a flexible wall portion with an outer side disposed opposite said delivery duct; a stop against which said outer side bears when a pressure in said delivery duct lies in a predefined operating pressure range; a pump disposed in said delivery duct upstream of said flexible wall portion in a liquid additive delivery direction; and a spring element disposed on said outer side of said flexible wall portion and configured to deform said flexible wall portion to reduce a size of said overall volume of said delivery duct when a pressure in said delivery duct is lower than a threshold pressure.
 2. The delivery device according to claim 1, which further comprises a return line branching off from said delivery duct downstream of said pump in said delivery direction, and a return valve configured to close off said return line.
 3. The delivery device according to claim 1, which further comprises at least one force sensor disposed on said stop and configured to measure a force exerted on said stop by said flexible wall portion to determine a pressure in said delivery duct.
 4. The delivery device according to claim 1, wherein said pump has a pump outlet, said delivery duct has a chamber downstream of said pump outlet in said delivery direction, said pump outlet opens into said chamber, and said flexible wall portion is disposed on said chamber opposite said pump outlet.
 5. The delivery device according to claim 1, wherein said stop has a receptacle configured to accommodate said spring element when said flexible wall portion bears against said stop.
 6. The delivery device according to claim 1, wherein said flexible wall portion has a surface area, and said spring element exerts a spring force on said flexible wall portion corresponding to a pressure of 0.2 to 1.0 bar in relation to said surface area of said flexible wall portion when said flexible wall portion bears against said stop.
 7. A method for compensating a formation of ice in a delivery device having a delivery duct with an overall volume at least partially filled with liquid additive, the method comprising the following steps: providing a pump in the delivery device; providing a return line with a return valve between the delivery duct and a tank for the liquid additive; a) stopping operation of the pump; b) opening the return valve of the return line producing a connection between the delivery duct and the tank; c) actively reducing a size of the overall volume filled with the liquid additive; d) discharging the liquid additive through the return valve into the tank; and e) passively increasing the overall volume as the liquid additive in the delivery duct freezes.
 8. The method according to claim 7, which further comprises carrying out step c) by pressing a spring against a flexible wall portion into a chamber of the delivery duct.
 9. A motor vehicle, comprising: an internal combustion engine; an exhaust-gas treatment device for purification of exhaust gases of said internal combustion engine; a tank for a liquid additive; and a delivery device according to claim 1 configured to deliver liquid additive from said tank to said exhaust-gas treatment device. 